This invention relates to a treatment for removing transition group metal impurities from silicon semiconductor material. It is particularly concerned with the removal of such impurities before and during device fabrication.
The detrimental effect of transition group metal impurities is well known and results from a complex interaction between the silicon lattice and the metal impurities, especially during high temperature heat treatments in oxygen or steam. Presence of the impurities during oxidation causes or enhances the formation of oxidation induced stacking faults in the silicon surface. These stacking faults then in turn are decorated by the metal impurities attracted by these crystal defects; the decorated defects then act as carrier recombination-regeneration centers and degrade performance of devices located in the silicon surface. It is therefore important that the silicon should be completely clean at the time the first thick layer of oxide is grown on a wafer.
The transition metal impurities include among others, copper, gold, iron, cobalt, nickel, chromium, tantalum, zinc and tungsten. These metals, even in small quantities, introduce defect sites in semiconductor material which can ultimately result in degraded device performance. The prior art is replete with techniques developed through the years for avoiding or removing such impurities in silicon semiconductor material particularly. These techniques range from "clean handling" to various gettering methods using doped glasses and other coatings, and by the use of damaged surface areas, induced by various techniques, which later are removed.
U.S. Pat. No. 3,556,879 discloses a heat treatment in an atmosphere of hydrogen chloride. It is known, as disclosed in the above-noted patent, that the metal impurities referred to hereinabove, for the most part, form volatile halides so that the reactive halide treatment constitutes an effective removal technique. However, in the above-noted patent, the hydrogen chloride etching treatment, is done in a water-vapor atmosphere, and therefore is accompanied by the formation at the outset of the process, or even before exposure to hydrogen chloride, of a relatively thick film of silicon dioxide. Our own work indicates that the effect of the hydrogen chloride is negated by the simultaneous presence of water-vapor, but not by the presence of dry oxygen.
The use of a silicon dioxide film for protection of a silicon semiconductor surface during heat treatment in a corrosive ambient in order to prevent erosion and pitting of the surface is well known. As suggested in the reference patent, this is accomplished by the provision of a silicon dioxide film of a thousand or more Angstroms in thickness which may be thermally grown or, as suggested alternatively by the reference patent, may be pyrolytically deposited.
Although gettering techniques have been widely practiced, metals of the transition group continue to be a problem as a consequence of their presence even in carefully treated silicon semiconductor material. Stacking faults produced as a consequence of the presence of these impurities during oxidation result in crystalline defects which affect the hold times of MOS memory devices as well as the switching characteristics of other silicon semiconductor devices.
A powerful measurement tool, neutron activation analysis, enables determination of extremely small amounts of impurity atoms such as the transition metals. Use of neutron activation analysis at the various stages of semiconductor device fabrication and before and after particular cleaning steps, reveals both the possible sources of contamination and the effectiveness of the cleaning processes. The results set forth in this disclosure are based, to a large extent, on analyzing impurity content before and after treatment by neutron activation analysis.
Accordingly, it is an object of this invention to remove, more effectively, impurity atoms of certain of the transition metals from silicon semiconductor material, particularly in the earlier stages of device fabrication.